The present invention relates generally to Electronic Surveillance Measurement (ESM) receivers such as Instantaneous Frequency Measurement (IFM) receivers, and more particularly relates to the selection of delays in IFM receivers to minimize the quantity of delay lines required to achieve a given accuracy.
Electronic Surveillance Measurement (ESM) receivers commonly require that frequency calculations be performed on an input signal from targets of interest. The frequency of the input signal is often measured using an Instantaneous Frequency Measurement (IFM) receiver as illustrated in FIG. 1. The IFM receiver uses a difference in phase between a delayed and a non-delayed version of the input signal to calculate the frequency of the input signal.
The IFM receiver is the simplest, most mature technique for obtaining accurate pule-by-pulse frequency information over a broad frequency band. Accurate frequency information is required for a variety of purposes in ESM receivers. For instance, it is useful as a sorting parameter for de-interleaving multiple emitters in a dense environment, being second only to angle-of-arrival in the hierarchy of sorting parameters. An additional role is in active power-managed ECM systems to define the band of jamming response, thereby optimizing utilization of jamming power and maximizing jamming effectiveness. Further, IFM receivers are useful in the detection and display of frequency-agile and pulse compression radar emitters.
The IFM receiver illustrated in FIG. 1 includes a receptor element or antenna 10, a power divider 22, a reference delay line 12, one or more differential delay lines 14, an N-channel phase measurement receiver 16, and a phase-to-frequency decoder 18. An input signal 20 is received and split into two or more constituent signals by the power divider 22. One constituent signal is applied to the reference delay line 12, and the remaining constituent signals are applied to the differential delay lines 14. The delayed signals are then applied to separate channels of the N-channel phase measurement receiver 16. The difference in delay between the reference delay line 12 and the differential delay lines 14 causes a frequency dependent phase shift, which is measured by the N-channel phase measurement receiver 16. The frequency of the input signal 20 is determined from this phase shift by the phase-to-frequency decoder 18.
Phase measurement receivers are alternatively referred to as phase discriminators, phase correlators or quadrature mixers. Further detail regarding phase measurement receivers can be found in a product specification catalog entitled, Anaren RF and Microwave Components, February 1997, distributed by Anaren Microwave, Inc., 6635 Kirkville Road, East Syracuse, New York 13057, the pertinent portions of which are incorporated herein by reference.
The phase shift or difference "psgr" radians (relative to the path through the reference delay line 12), which is created by a delay difference xcfx84 seconds (relative to the same path through the reference delay line 12) at a frequency f hertz, is given by equation (1) as follows:
"psgr"=2xcfx80xcfx84f,xe2x80x83xe2x80x83(1)
and thus
f="psgr"/2xcfx80xcfx84,xe2x80x83xe2x80x83(2)
where xcfx84=xcex941/c is the delay difference in seconds xcex941 is a length differential between the delay lines, and c is a velocity of propagation in the delay lines.
A measured phase difference xcfx86meas will differ from the phase difference "psgr" due to measurement error and the inability to measure phase values outside a range of 2xcfx80. For example, assume that the phase is measured with a phase receiver having a random phase measurement error xcex5 degrees at a particular signal-to-noise ratio, and that bias error is removed by calibration. The measured phase difference xcfx86meas that results is given by equation (3) as follows:
xcfx86meas=MOD2xcfx80(2xcfx80xcfx84f+E),xe2x80x83xe2x80x83(3)
where E is a random variable with a standard deviation equal to the phase measurement error xcex5 and a mean of zero.
Using a large value for the delay difference xcfx84 produces a large phase difference slope with frequency, which minimizes the phase measurement error xcex5 caused by E. This translates to a more accurate determination of frequency. Specifically, with a phase receiver having the phase measurement error xcex5 degrees, a frequency error xcex94f hertz, standard deviation, is given by equation (4) as follows:
xcex94f=(xcex5/360)(1/xcfx84).xe2x80x83xe2x80x83(4)
For example, with the delay difference xcfx84 equal to 5 nanoseconds, and the phase measurement error xcex5 equal to 7.2 degrees, the IFM receiver is capable of a frequency measurement with the frequency error xcex94f equal to 4 MHZ.
However, xcfx86meas, and thus a measured frequency fmeas, is ambiguous for values of xcfx84f which are greater than unity because phase can only be determined nonambiguously within a range of 2xcfx80. In fact, multiple ambiguities will be spaced at intervals of frequency equal to 1/xcfx84. For the example above, xcfx84f is greater than unity for f greater than 0.2 Ghz, and ambiguities occur every 0.2 Ghz. In other words, the output of the phase receiver is identical for input frequencies of 0.05 Ghz, 0.25 Ghz, 0.45 Ghz, and so forth.
To measure frequency nonambiguously with one differential delay, a nonambiguous delay xcfx84nonamb must be chosen such that it is equal to or less than 1/fmax, where fmax is a maximum frequency to which the phase receiver will be subjected. To reduce the number of ambiguities in a practical multi-channel IFM receiver, as shown in FIG. 2, the input signal 20 is translated to an IF frequency band by a frequency converter 24 to limit fmax. Such a configuration operates on input signals 20 in one frequency band at a time, and switches local oscillators within the frequency converter 24 to change bands. A typical IF band is about 2-4 Ghz, and thus fmax is typically about 4 Ghz.
To resolve ambiguities, IFM receivers commonly perform a second nonambiguous measurement using delay lines configured in binary ratios such as 1, 2, 4 and 8 as shown in FIG. 3. The nonambiguous delay xcfx84nonamb is the shortest delay used in such a Binary IFM receiver (BIFM), and it produces unambiguous estimates for the most significant frequency bit. The next differential delay is twice the nonambiguous delay xcfx84nonamb. This next differential delay produces a frequency estimate with twice the accuracy, which is used for the next most significant frequency bit, and it can have one ambiguity which can be resolved with the prior nonambiguous delay xcfx84nonamb. This process can be continued in binary fashion by adding additional differential delays, each twice as long as the preceding one, so that the length of the ith delay is given by equation (5) as follows:
xcfx84BIFMi=2(ixe2x88x921)/(fmax),xe2x80x83xe2x80x83(5)
and a frequency estimation error xcex94fBIFM of the ith delay for the BIFM is given by equation (6) as follows:
xe2x80x83xcex94fBIFMi=fmax(xcex5/360)(xc2xd(ixe2x88x921)),xe2x80x83xe2x80x83(6)
where
i=1+ceil {log(xcex5fmax/360xcex94fBIFM)}/log(2),xe2x80x83xe2x80x83(7)
and ceil refers to a xe2x80x9cceilingxe2x80x9d function which outputs the next integer greater than its operand. For instance, ceil (5.2)=6, ceil (11.3)=12, ceil (10.5)=11, and so forth.
Thus, for the frequency error xcex94f of no more than 5 MHZ with a phase receiver having a phase measurement error xcex5 equal to 7.2 degrees and a maximum frequency equal to 4 Ghz, i must be at least 5. That is, the BIFM must have at least a six channel phase receiver with one reference delay line and five differential delay lines.
BIFM receivers have also been implemented with delay lines differing in length by a factor of 4 rather than 2 as described above. Such receivers require fewer delay lines. For instance, using the example above, only 3 differential delay lines are required for the frequency error xcex94f equal to 5 MHZ.
The quantity of delay lines required depends upon the accuracy desired for the frequency measurement. However, the cost and complexity of IFM receivers increases as the number of delay lines increases. Thus, it would be advantageous if the number of delay lines required to achieve a given accuracy in frequency measurement could be reduced from that required by conventional BIFM receivers without substantially degrading the accuracy of the frequency measurement.
It is an object of the present invention to provide an Instantaneous Frequency Measurement (IFM) receiver for calculating a frequency of input signals using fewer delay lines to achieve a given accuracy than conventional IFM receivers.
It is a further object of the present invention to provide an IFM receiver, which is less costly and complex to manufacture and smaller than conventional IFM receivers.
It is yet a further object of the present invention to provide an IFM receiver which requires fewer channels from an N-channel phase receiver to achieve a given accuracy than conventional IFM receivers.
It is another object of the present invention to provide a method of selecting delay lines in IFM receivers which minimizes the quantity of delay lines required to achieve a given accuracy in measuring frequency.
In accordance with one form of the present invention, an IFM receiver is provided, which receives input signals from a target and determines the frequency of the signals. The IFM receiver includes delay lines configured in lengths which either forms relatively prime ratios, or which are selected according to the following relationship:
xcfx84n=((360/4xcex5)xe2x88x921)nxe2x88x921(1/fmax),xe2x80x83xe2x80x83(17)
where xcfx84 is the delay associated with a delay line in seconds, n is an integer indexing variable greater than or equal to 1 which refers to a particular delay line, xcex5 is a phase measurement error, and fmax is a maximum frequency to which an n-channel phase receiver will be subjected. The input signal is fed to the delay lines, which provide delayed versions of the input signals to separate channels of an N-channel phase receiver. The N-channel phase receiver determines phase information representing phase difference between the delayed versions of the input signal. The output of the N-channel phase receiver is provided to a phase translation circuit, which translates the phase information into frequency information indicative of the frequency of the signal from the target.
Previously, IFM receivers included delay lines configured in binary ratios. By implementing an IFM receiver with delay lines configured in lengths according to the relationships provided above, substantially the same accuracy can be achieved with fewer delay lines. A reduction in delay lines reduces the cost, complexity and size of the IFM receiver.
In accordance with another form of the present invention, methods of selecting delay lines in IFM receivers is provided. The methods are based on delays which either selected in relatively prime ratios, or are selected according to the following relationship:
xcfx84n((360/4xcex5)xe2x88x921)nxe2x88x921(1/fmax),xe2x80x83xe2x80x83(17)
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.